1. Field of the Invention
The present invention relates to liquid crystal display using ferroelectric liquid crystal, more specifically, liquid crystal display in which a photo alignment layer is used to control the alignment of ferroelectric liquid crystal.
2. Description of the Related Art
Since liquid crystal display has features that it is thin and is low in power consumption and other features, the use thereof has been expanding in various articles from large-sized displays to portable information terminals and the development thereof has been actively made. Conventionally, for liquid crystal display, a TN system, an STN multiplex driving system, an active matrix driving system in which thin film transistors (TFT) are used in TN, and others have been developed and made practicable. However, nematic liquid crystal is used therein; therefore, the response speed of the liquid crystal material is as small as several milliseconds to several tens of milliseconds and it cannot be said that these sufficiently cope with display of moving images.
Ferroelectric liquid crystal (FLC) exhibits a very short response time in order of microseconds, and thus FLC is a liquid crystal suitable for high-speed devices. About ferroelectric liquid crystal, there is well known a bistable liquid crystal which has two stable states when no voltage is applied thereto and is suggested by Clark and Lagerwall (FIG. 4). However, the liquid crystal has a problem that the liquid crystal has memory property but graduation display cannot be attained since the switching thereof is limited to two states, namely, bright and dark states.
In recent years, attention has been paid to ferroelectric liquid crystal in which the liquid crystal layer thereof is stable in a single state (hereinafter referred to as “monostable”) when no voltage is applied thereto as a liquid crystal making it possible to attain graduation display by the matter that the director (the inclination of the molecule axis) of the liquid crystal is continuously changed by a change in applied voltage so as to analogue-modulate the light transmission thereof (NONAKA, T., LI, J., OGAWA, A., HORNUNG, B., SCHMIDT, W., WINGEN, R., and DUBAL, H., 1999, Liq. Cryst., 26, 1599., FIG. 4).
As the liquid crystal showing the mono-stability, in general, a ferroelectric liquid crystal having the phase change of cholesteric phase (Ch)-chiral smectic C (SmC*) phase without the transition to the smectic A (SmA) phase in the temperature cooling process is used.
On the other hand, as the ferroelectric liquid crystal, here is a material having the phase change of Ch-SmA-SmC* so as to show the SmC* phase via the SmA phase in the temperature cooling process. Among the ferroelectric liquid crystal material reported so far, most of them are those having such a phase sequence compared with the former material without the transition to the SmA phase. It is known that such a ferroelectric liquid crystal having such a phase sequence in general has two stable states with respect to one layer normal line so as to show the bi-stability (FIG. 5).
As a method for processing such ferroelectric liquid crystal for providing the mono-stability, a polymer stabilizing method can be presented. The polymer stabilizing method is a stabilization method by injecting a ferroelectric liquid crystal mixed with an ultraviolet curable monomer in a liquid crystal cell with the alignment process applied, and executing the ultraviolet ray irradiation in a state with the direct current or alternative current voltage applied for the polymerization. However, a problem is involved in that the production process is complicated and the driving voltage is made higher.
Moreover, in general, as the technique for subjecting liquid crystal to alignment process, there is known a method of using an alignment film. The method is classified into the rubbing method and the photo alignment method. The rubbing method is a method of subjecting a substrate coated with a polyimide film to rubbing treatment to align chains of the polyimide polymer in the direction of the rubbing, thereby aligning liquid crystal molecules on the film. The rubbing method is excellent in controllability of the alignment of nematic liquid crystal, and is generally an industrially applicable technique. However, according to this method, there are problems of the static electricity or dust generation, unevenness of the alignment limiting force or the tilt angle due to the rubbing condition difference, irregularity at the time of the large area process, or the like. Compared with the nematic liquid crystal, due to the high molecule order, the alignment control is difficult, and thus it is not suitable for the alignment processing method for the ferroelectric liquid crystal, which can easily generate the alignment defect.
As a non contact alignment method to take place of the above-mentioned rubbing method, a photo alignment method can be presented. The photo alignment method is for aligning the liquid crystal molecule of the film by directing a light beam with the polarization control to a polymer or a monomolecule for generating the photo excitation reaction (decomposition, isomerization, dimerization) so as to provide the anisotropy to the polymer film or the monomolecular film. This method is advantageous in that the quantitative alignment process can be controlled without generation of the static electricity or the dusts, which is the problem of the rubbing method. However, even by using this method, an example of realizing the mono-stability operation mode using the ferroelectric liquid crystal essentially having the bi-stability has not been found so far.
On the other hand, since the memory property is not provided in the case the ferroelectric liquid crystal shows the mono-stability, an active matrix drive wherein an active element such as a transistor and a diode is provided for each pixel is suitable. In particular, in the case an active matrix system using a TFT element as the active element is employed, since the purposed pixel can certainly be turned on or off, a high quality display can be realized, and thus it is advantageous.
In recent years, color liquid crystal display has been actively developed. The method for realizing color display is generally classified into a color filter system and a field sequential color system. The color filter system is a system of using a white light source as a back light and attaching a micro color filter in R, G or B color to each pixel, thereby realizing color display. On the other hand, the field sequential color system is a system of switching a back light into R, G, B, R, G, B . . . with time, and opening and shutting a black and white shutter of a ferroelectric liquid crystal in synchronization therewith to mix the colors with time by afterimage effect on the retina, thereby realizing color display. This field sequential color system makes it possible to attain color display in each pixel, and does not require any color filter low in transmission. As a result, this system is useful since the system is capable of attaining bright and highly precise color display and realizing low power consumption and low costs.
However, the field sequential color system is a system in which each pixel is subjected to time sharing; it is therefore necessary for the liquid crystal as the black and white shutter to have high-speed responsiveness in order to give a good moving image display property. If ferroelectric liquid crystal is used, this problem can be solved. However, the ferroelectric liquid crystal has a problem that alignment defects are easily generated, as described above. Furthermore, as the ferroelectric liquid crystal, those showing the mono-stability operation mode are desirable in order to enable the graduation display by the analog modulation and realize the highly precise color display as mentioned above. However, the ferroelectric liquid crystal materials showing the mono-stability are limited. Due to the narrow material selection range of the ferroelectric liquid crystal, it is difficult to cope with the various demanded characteristics, and thus they have not been provided for the practical use so far.